This makes the CEGUI dependency optional and off by default. Restoring
previous behaviour of exiting on disconnect if disabled.

Also prints a simpler error to stderr for WARN or above so that
we print something on the commandline if something go wrong.

The OpenGL renderer isn't really useful right now, its not quite up
to date, its not really faster than software and it only supports a limited
subset of drivers. So, lets disable it for now.

Long term opengl rendering of the 2d part of spice is important if we want
to combine 2d and 3d rendering (say if spice adds opengl support in the
protocol). But until then this is isn't useful for normal use.